The functional unit of the human kidney is the nephron. There are about one million nephrons present in the cortex of each kidney and together they are responsible for the purification and filtration of the blood. As blood flows through the kidneys, the nephrons determine the amount of water, salts, glucose, urea and other minerals it contains. When excess substances are present, the surfeit quickly passes out in the urine. However, the kidneys accelerate their reclamation of these same substances when they are present in the blood in amounts that are inadequate.